1. Field of the Invention
This technology relates to programming of a nonvolatile memory array.
2. Description of Related Art
The threshold voltage of a nonvolatile memory cell is determined generally by the variable amount of charge which is stored in the charge storage layer of each nonvolatile memory cell. The sensing window of a memory must be sufficiently wide to allow for margin between the different threshold voltage states—such as fully programmed, fully erased, and (for multi-level cells) various intermediate states—so that a read operation can reliably distinguish between the differently programmed and erased threshold voltage states. However, different memory cells in a memory array will have varying electrical characteristics, such as resistance and capacitance, depending on the position of the memory cell in the memory array, and depending on the fabrication parameters at particular positions in the memory array. Despite such variations between individual memory cells, the sensing window must nevertheless be sufficiently wide to distinguish between the different threshold voltage states, regardless of the particular physical position of the memory cell in the array.
The “second bit effect” is yet another source of variation among the memory cells of an array which reduces the sensing window. The “second bit effect” refers to the disturbance in SONOS cells storing data on two ends of the channel, of a left end bit by a program operation on the right end bit. U.S. Pat. No. 6,011,725 (Eitan), the entire content of which is incorporated by reference herein, provides a detailed comparison of several of the prior art nonvolatile memories including respective programming, erasing and reading techniques, and the related second bit effect. The Eitan patent also discloses a type of silicon-oxide-nitride-oxide-silicon (SONOS) memory cell capable of storing two data bits by localized charge storage techniques.
Stated differently, the “second bit effect” refers to the reverse read current of an unprogrammed bit on one side of the memory, where the reverse read current must “punch through” the channel region under a programmed bit on the other side of the memory. The second bit effect narrows the operation sensing window, of the difference between the threshold voltages of a programmed bit on one side of the memory and an unprogrammed bit on the other side of the memory. Because the different memory cells of an array have different amounts of programming, the “second bit effect” also has different effects among different memory cells.
Consequently, the sensing window is defined by various considerations, such as variable stored charge, margin, and the effects described above.